A front derailleur of the type described above is illustrated in FIG. 5 of U.S. Pat. No. 5,470,277 to Romano, and in FIG. 11 of Italian Patent Application no. TO99A001023 to Valle. FIG. 1 of the accompanying drawings, provided for quick reference, shows a front derailleur of the type presented in the Valle application. In this figure, the front derailleur is generally indicated with reference numeral 1. It comprises a supporting body 2 with a band 3 for fastening to a tube of a bicycle frame. Numeral 5 generally refers to a fork unit, of the known type, for chain derailing (not illustrated) of the bicycle for the selective engagement of said chain with the sprockets (not shown) associated to the central movement of the bicycle. The fork unit 5 is connected to the supporting body 2 by means of an articulated parallelogram system, comprising two arms 6, 7 respectively pivoting in 8, 9 and 10, 11 to the supporting body 2 and to the fork unit 5. The articulated arm 7 arranged on the external side extends to actuating arm 12, forming a single part with arm 7 at an angle of approximately 90°. A worm screw element 13, fastened onto a screw 14, connected to the output shaft of an electrical motor 15 of the front derailleur, pivots on the arm 12. The electrical motor 15 is equipped with a band 16 for connecting the motor body in a pivoting fashion on an axis 17 of the derailleur 2, because the motor must be able to oscillate on the axis 17 when, following operation, the arm 12 makes an angular oscillation on the articulation on the support unit 2.
Said solution presented several problems, however. Firstly, the electrical motor must make a high number of turns to move the fork unit between its two extreme positions. Secondly, as mentioned, due to the system geometry, the motor 15 must be fitted to pivot on axis 17, and operatively occupies a relatively large space, with the additional problem of possible coupling play. Finally, with the geometry illustrated in FIG. 1, a wide maximum excursion of the front derailleur cannot be obtained if the dimensions are to be kept relatively low, without seizing. This is required, for example, in the cases in which three sprockets are associated to the central movement.